Conventionally, a steam generator is configured to have a first tank used as a liquid supply source, a second tank which is a vaporizing chamber, a liquid supply pipe which supplies liquid from the first tank to the second tank, and a liquid discharge pipe which discharges liquid from the second tank to the first tank. In the conventional steam generator having the above-mentioned configuration, a cleaning operation is performed to remove scales adhered to an inside of the second tank. During the cleaning operation, liquid is supplied to the second tank until a liquid level of the second tank reaches a level which exceeds an upmost point of the bended liquid discharge pipe, and then the liquid in the second tank is discharged to the first tank according to the principal of a syphon, and thereby discharging the scales in the second tank (refer to Unexamined Japanese Patent Publication No. 2010-54096 (Patent Literature 1)). While a material of the liquid discharge pipe is not clearly described, one formed by bending a metallic pipe is generally used for such liquid discharge pipe.